CodeIgniter’s Hooks feature provides a means to tap into and modify the
inner workings of the framework without hacking the core files. When
CodeIgniter runs it follows a specific execution process, diagramed in
the Application Flow page. There may be
instances, however, where you’d like to cause some action to take place
at a particular stage in the execution process. For example, you might
want to run a script right before your controllers get loaded, or right
after, or you might want to trigger one of your own scripts in some
other location.

The array index correlates to the name of the particular hook point you
want to use. In the above example the hook point is pre_controller. A
list of hook points is found below. The following items should be
defined in your associative hook array:

class The name of the class you wish to invoke. If you prefer to
use a procedural function instead of a class, leave this item blank.

function The function (or method) name you wish to call.

filename The file name containing your class/function.

filepath The name of the directory containing your script.
Note:
Your script must be located in a directory INSIDE your application/
directory, so the file path is relative to that directory. For example,
if your script is located in application/hooks/, you will simply use
‘hooks’ as your filepath. If your script is located in
application/hooks/utilities/ you will use ‘hooks/utilities’ as your
filepath. No trailing slash.

params Any parameters you wish to pass to your script. This item
is optional.

If you’re running PHP 5.3+, you can also use lambda/anoymous functions
(or closures) as hooks, with a simpler syntax:

pre_system
Called very early during system execution. Only the benchmark and
hooks class have been loaded at this point. No routing or other
processes have happened.

pre_controller
Called immediately prior to any of your controllers being called.
All base classes, routing, and security checks have been done.

post_controller_constructor
Called immediately after your controller is instantiated, but prior
to any method calls happening.

post_controller
Called immediately after your controller is fully executed.

display_override
Overrides the _display() method, used to send the finalized page
to the web browser at the end of system execution. This permits you
to use your own display methodology. Note that you will need to
reference the CI superobject with $this->CI=&get_instance() and
then the finalized data will be available by calling
$this->CI->output->get_output().

cache_override
Enables you to call your own method instead of the _display_cache()
method in the Output Library. This permits
you to use your own cache display mechanism.

post_system
Called after the final rendered page is sent to the browser, at the
end of system execution after the finalized data is sent to the
browser.